The problem of waste heat recovery, i.e. transformation into power, is an important factor with various geothermal heat sources. Only a few, such as located at Larderello in Italy, the geysers in California, and certain artificial wells in which water is injected in hot rock, are at sufficiently high temperature to produce steam which is dry and at a temperature and pressure suitable for direct use in steam turbines. In many other localities, hot water or hot water and steam are produced.
Other sources of waste heat include flue gases from power plants and incinerators and process gases of chemical and metallurgical operations. Waste heat boilers have been used for energy recovery from these sources. Heat is transferred from the source to the working fluid in the boiler, causing evaporation and producing steam which is expanded to generate shaft energy. In this method very little energy is recoverable below the temperature of the boiler (in economizers) and very little is recoverable at a level of temperature significantly above the boiler temperature (superheaters and reheaters). Thus a major part of the thermal energy is either lost or degraded so that the efficiency of conversion to shaft work is low.
To overcome this deficiency a multiplicity of waste heat boilers may be employed to utilize the thermal energy at more than one level of temperature to reduce the loss and/or degradation. It has also been proposed in a series of patents to Turner, of which U.S. Pat. No. 2,151,949 is typical, to utilize the sensible heat of pressurized water by converting this into kinetic energy which is utilized in impingement on the moving buckets of an impulse turbine producing shaft energy.
In Turner the heated pressurized water is expanded by reducing the pressure to cause a fractional adiabatic self-evaporation. This results in an acceleration of the mixture of vapor and liquid. Streams of the two-phase working fluid pass through nozzles and impinge on the buckets of the turbine. Other similar proposals have been made, e.g., in an article of Basil Wood, UNESCO Earth Sciences, Vol. 12, p. 118, relating to geothermal energy.
The present invention deals with an improved process and system in which waste heat is transferred or is present in a pressurized working fluid which is expanded in multiple stages to achieve a highly efficient conversion to shaft work.